Gram positive bacteria, primarily streptococci and actinomyces, initiate oral microbial colonization, subsequent dental plaque formation and gingival inflammation. These biological events, which represent essential steps in the development of caries and periodontal diseases, depend on specific adhesive properties, that are common to a number of oral bacteria. One such interaction is the adhesion of bacteria to the acquired pellicle, an interaction that initiates the colonization of teeth by viridans streptococci. Extensive previous studies have shown that this process involves a sialic acid reactive lectin activity and that this activity also mediates bacterial adhesions to various host cells including erythrocytes. The lectin activity of Streptococcus gordonii Challis has now been associated with a specific bacterial cell surface antigen. This involved the antigenic comparison of strain Challis with a spontaneous mutant that specifically lacked hemagglutinating activity. Crossed immunoelectrophoresis revealed a native antigen present in extracts of the parent strain that was absent in extracts of the nonadherent mutant. The purified antigen was composed of both protein and carbohydrate and had a fimbrial morphology based on the appearance of cells labeled with monospecific antibody prepared against the purified antigen. Significantly, Fab fragments of this antibody inhibited lectin-mediated bacterial hemagglutination, thereby associating the sialic acid binding lectin activity with a specific bacterial cell surface structure. Streptococcal cell wall polysaccharides may also play an essential role in oral microbial colonization since these molecules are recognized by lectins on other bacteria. Structural, immunochemical and genetic studies of these streptococcal polysaccharides are underway to more clearly define the determinants of immunological and lectin recognition. Insight concerning the molecular basis of various oral microbial adhesive interactions may well suggest novel approaches for the control of plaque related diseases.